Photo: WB Television Network In the final moments of Buffy the Vampire Slayer ’s fifth-season premiere, the titular slayer (Sarah Michelle Gellar) heads up to her room to grab her things before going on a date with her hunky college boyfriend, Riley (Marc Blucas). Standing in Buffy’s sunlit bedroom is Dawn (Michelle Trachtenberg) , her little sister, a scenario otherwise completely innocuous except that, for the first four years of the show, Buffy doesn’t have a little sister. Dawn’s retconning